The slogan on the bus in Istanbul reads "We do not forget July 15" - the date of the attempted coup in 2016. But Turkey's has wreaked a ruthless revemnge with Human Rights Watch claiming widespread torture of detainees in the past year, and especially those accused of any invovlvment with Fethullah Gulen, the man the government blames for the coup attempt. Ozan Kose / AFP

Human Rights Watch on Thursday claimed there was growing evidence of detention abuses in Turkey after last year's failed coup, warning that torture in police custody had become a "widespread" problem.

The US-based watchdog cited "credible evidence" of 11 cases of serious abuse including severe beatings, sexual assault or the threat of sexual assault and being stripped naked. But it said the 11 cases represented a fraction of the credible narratives reported in the media and on social media.

"Such reports indicate that torture and ill-treatment in police custody in Turkey has become a widespread problem," HRW said in its latest report.

The alleged victims are suspects accused of links to terror organisations or to organisations or individuals which the Turkish authorities believe to be linked to the attempted coup.

Some detainees reported their ill-treatment to prosecutors or during court hearings, but HRW says the complaints were not investigated "effectively".

The group accused Ankara of failing to act to stamp out "abusive practices" which it claims have become far more common in police custody over the past year.

"As evidence mounts that torture in police custody has returned to Turkey, the government urgently needs to investigate and call a halt to it," Hugh Williamson, Europe and Central Asia director at HRW, urged in a statement.

HRW said victims were frightened to complain for fear of reprisals against their family.

— Abductions —

The group also said there were five cases of abductions in Ankara and the western city of Izmir between March and June "that could amount to enforced disappearances".

In one alleged case cited by HRW, Onder Asan, a former teacher, was "abducted" in April and remained missing for 42 days before he turned up in police custody and was then sent to pretrial detention.

The Turkish government has not commented so far.

Last month, justice minister Abdulhamit Gul said Turkey had "zero tolerance for torture", noting the government's commitment to human rights.

HRW said those most at risk of torture were suspects detained over alleged links to the coup-plotters or the outlawed Kurdistan Workers' Party (PKK).